The proposed research is an extension and expansion of our prospective, longitudinal study of math skills -development in primary school age children. The broad, long-term objectives of this project are to contribute toward understanding math ability, math disability (MD), and MD subtypes. Towards these objectives, the following specific aims are proposed: (1) To examine the stability of math skills in children with or without MD, from grades K to 5. Of interest is whether distinct profiles corresponding to Visuospatial, Semantic Memory, and Procedural MD subtypes proposed by Geary (1993) persist over time; (2) To address whether MD emerges in some students beyond grade 2, and if so, if it persists over time and is linked to a specific MD subtype; (3) To evaluate cognitive and behavioral correlates, including attentional components, of concurrent and later math performance; (4) To examine how children with fragile X or Turner syndrome serve as potential models of MD subtypes, in view of their risk for poor math achievement; and (5) to identify molecular-clinical correlates in children with fragile X or Turner syndrome. The health-relatedness of the project is its contribution to defining and identifying MD, and to finding correlates and possible core cognitive deficits of MD. Enhanced awareness of core deficits and MD subtypes will guide identification and intervention of MD. The research design involves the study of various groups at risk for MD, and of normally achieving children. The methods include: 1) continued testing of a school-based sample of 220 children (110 boys, 110 girls) through grade 5, to examine stability of math achievement and early predictors of MD in elementary school; 2) recruitment of 44 additional children with MD, to increase sample size for MD versus Non-MD group comparisons; and 3) testing a sample of children with fragile X or Turner syndrome, two groups at risk for poor math achievement, inclusion of normative and genetic models of MD will provide a unique perspective from which to examine the trajectory and normal variation in the development of MD and MD subtypes.